The present invention relates to a cylindrical permanent magnet having multipolar anisotropy and more particularly to a cylindrical permanent magnet having multipolar anisotropy at the surface with a large magnetic force but no danger of cracking.
Cylindrical permanent magnets having a large number of magnetic poles are in wide use in, for example, the rotor of stepping motor. Such magnets are known as cylindrical permanent magnets having anisotropy in a radial direction. Typical examples of these cylindrical permanent magnets include one made of ferrite (U.S. Pat. No. 4,010,434) and one made of a rare earth element and cobalt (Japanese Pat. Publication No. 59-23448).
Cylindrical permanent magnets having a large number of magnetic poles at the surface have recently been proposed as magnets having more magnetic poles and a larger magnetic force than the above-mentioned cylindrical permanent magnets having anisotropy in a radial direction, and are in practical use. For example, U.S. Pat. No. 4,547,758 discloses a sintered cylindrical permanent magnet having a composition of MO.nFe.sub.2 O.sub.3 (M is Ba, Sr or Pb and n is 5 to 6) and having at least 8 magnetic poles at the surface. In this magnet, the magnetic flux flows between magnetic poles in a form of circular arcs as shown in FIG. 1 and the particles having magnetic anisotropy are aligned substantially along the circular arcs; therefore, the magnet can have not only a remarkably increased pole density but also a larger magnetic force.
In order to further increase the magnetic force of the cylindrical permanent magnet having multipolar anisotropy at the surface attempts have been made, to form a sintered magnet using a rare earth element-cobalt alloy in place of ferrite. However, the resulting rare earth element-cobalt cylindrical permanent magnet having multipolar anisotropy at the surface have a tendency to form cracks. The reason for this cracking is believed to be that the sintered body has insufficient strength to withstand the thermal stress as generated during sintering.